A major gene controlling expression of infectious xenotropic murine leukemia viruses (X-MuLV) was mapped to chromosome 17 in linkage with H-2K. The effect of this locus appears to be restricted to the X-MuLV induction locus of F/St mice, located on chromosome 1. B10 mice congenic for the H-2 haplotypes of F/St, P/J, SM/J and Mus molossimus express high levels of infectious ecotropic and mink cell focus-forming (MCF) MuLV as a result of intrauterine and milk borne transmission of B-tropic ecotropic MuLV. Exposure to high levels of virus early in life result in premature graying and germ-line reintegrations of B tropic MuLV. Cas-Br-M, an ecotropic MuLV obtained from wild mice was shown to be unique among ecotropic MuLV in that it induces a wide spectrum of hematopoietic neoplasms including B cell, T cell, myelogenous, and erythroleukemias. In addition, a Cas-Br-M lymphoma was found to contain a unique spleen focus-forming virus.